Numerous fibrous filtration sheets have been prepared from a variety of fibers, such as glass, polyethylene, polypropylene, polyester, and aramid fibers. While these sheets are suitable for many applications, particularly many filtration applications, they are generally incapable of reliably removing small micron and submicron particulate matter and bacteria. While the thickness of such a fibrous sheet can be increased to compensate for the relatively large pore size of the sheet, the flow resistance of the sheet then becomes impractically high. This inability to prepare fibrous filtration sheets of sufficiently small pore size is generally the result of the unavailability of fibers of sufficiently fine diameter inasmuch as the pore size of a fibrous filtration sheet is related to some extent to the fiber diameter used to prepare such a sheet, i.e., uniformly smaller diameter fibers will more easily allow for the preparation of a fibrous filtration sheet of uniformly smaller pore size.
While many organic fibers, such as polyethylene and polypropylene, are available as staple cut fibers with diameters above about 8 microns, glass fibers are available with diameters of about 0.25-4 microns. Thus, glass fibers have generally been the material of choice in constructing fibrous filtration sheets of the finest pore sizes, albeit of insufficiently small pore size to remove very small particulate matter and bacteria.
While glass fibrous sheets are capable of providing the desirable smaller pore size as compared to polymeric fibrous sheets, glass fibrous sheets suffer from many performance disadvantages as compared to polymeric fibrous sheets. For example, glass fibrous sheets are relatively brittle and require extreme care in handling. As a result, glass fibrous sheets are difficult to convert into corrugated structures for filter elements without introducing cracks in the sheets, particularly in preparing small radii filter elements, with the consequent loss of filtration efficiency. The brittleness of glass fibrous sheets also render such sheets susceptible to failure upon pulsing of the fluid flow through the sheets. In addition, glass fibrous sheets are subject to significant chemical and thermal reactivity. For example, since glass is subject to degradation in alkaline environments, glass fibrous sheets are not well-suited for such environments. Moreover, glass fibrous sheets must be prepared using a resin binder, which is subject to chemical degradation, can adversely affect performance, and provides a potential source of contamination through leaching.
Accordingly, there remains a need for a fibrous filtration sheet with a relatively small pore size, preferably sufficiently small to remove submicron particles, especially bacteria, at a reasonable flow resistance through the fibrous sheet. Such a fibrous filtration sheet preferably would also have a high modulus, as well as sufficient elongation, so as to withstand filtration conditions, such as pulsing, which can be encountered in commercial filtration processes. The handling characteristics of such a fibrous filtration sheet should also be such that the sheet can be converted into a corrugated structure for a filter element without damaging the sheet in any way so as to affect filtration efficiency. A high temperature resistance and good chemical stability are other useful and desirable characteristics of such a fibrous filtration sheet.
The present invention provides such a fibrous filtration sheet, as well as a method of preparing and using such a fibrous filtration sheet, particularly in filtration applications. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.